The proton conductivity of SPEEK membranes in situ cross-linked by thermal treatment at 180 degrees C for various times was investigated by impedance spectroscopy. The conductivity measurements were made on fully humidified membranes between 25 and 65 degrees C and on membranes exposed to different relative humidity between 80 and 140 degrees C. The Ionic Exchange Capacity (IEC) was determined by acid-base titration and the water uptake by gravimetry. The proton conductivity was determined as function of temperature, IEC, degree of cross-linking and hydration number. A curve of proton conductivity vs. hydration number allows predicting that in order to reach a value of 0.1 S/cm at 100 degrees C a hydration number above 20 is necessary. The measured conductivity at this temperature is 0.16 S/cm for a hydration number of 60. (C) 2013 Elsevier B.V. All rights reserved.